Multimodal polyethylene copolymer

ABSTRACT

The present invention is directed to a multimodal polyethylene copolymer comprising a first and a second copolymer of ethylene and at least two alpha-olefin comonomers. The multimodal polyethylene copolymer is highly suitable for the production of films providing high impact strength in the sense of high Dart drop impact strength (DDI) and low extractables. The invention further presents final articles such as films made therefrom.

FIELD OF THE INVENTION

The present invention is directed to a multimodal polyethylene copolymer comprising a first and a second copolymer of ethylene and at least two alpha-olefin comonomers, highly suitable for the production of films providing high impact strength in the sense of high Dart drop impact strength (DDI) and low extractables. The invention further presents final articles such as films made therefrom.

BACKGROUND INFORMATION

Producing multimodal polyethylene copolymers is well known in the art. There are several various settings how to produce polyethylene grades having different density ranges, specific viscosities or comonomer compositions.

DESCRIPTION OF THE PRIOR ART

It is known to produce ethylene copolymers suitable for producing films by copolymerizing ethylene in two polymerization stages, for instance from EP-A-691367 which discloses bimodal ethylene copolymers produced in two fluidized bed reactors. The document does not disclose a third polymerization stage.

Also EP-A-773258 discloses blends of bimodal LLDPE resins with unimodal LLDPE. The bimodal LLDPE preferably has a density of from 0.91 to 0.930 g/cm³, an HLMI of 45 to 145 g/10 min and melt flow ratio HLMI/MI of 50 to 150 (p. 5, lines 38-42). The example discloses a density of 0.921 g/cm³, HLMI of 122 g/10 min and HLMI/MI of 99.

WO-A-97/50093 discloses bimodal LLDPE resins suitable for cable insulation. In examples it discloses a Copolymer E which was reported to be a bimodal LLDPE having a density of 0.913 g/cm³, MI of 0.6 g/10 min and HLMI of 50 g/10 min. The document is silent as to the manufacturing method.

EP-A-2067799 discloses multimodal LLDPE resins which have been produced in two polymerization stages in a loop and a gas phase reactor in the presence of a ligand-modified catalyst.

EP-A-2186833 discloses a three-stage polymerization in a cascaded reactor sequence of two loop reactors followed by a gas phase reactor. In the first stage a polymer having an MFR₂ of preferably 200 to 1000 g/10 min and a density of preferably 945 to 978 kg/m³ was produced. The polymer produced in the second stage had an MFR₂ of preferably 200 to 1000 g/10 min and a density of preferably 945 to 978 kg/m³. The final polymer had an MFR₂₁ of preferably 5 to 30 g/10 min and a density of preferably 940 to 970 kg/m³. The polymers produced in the first and second stages had the same MFR₂. In the exemplified process the polymers produced in the first two stages were homopolymers and the final resins had MFR₅ of from 0.2 to 0.4 g/10 min and density of about 955 kg/m³. The final polymer is comprised of only one alpha-olefin comonomer.

WO2009/071323 describes a multimodal linear low density polyethylene polymer having a final density of 900 to 940 kg/m³, and containing at least one α-olefin comonomer in addition to ethylene comprising: (A) less than 41 wt. % of a lower molecular weight component being an ethylene homopolymer or a copolymer of ethylene and at least one α-olefin; and (B) more than 59 wt. % of a higher molecular weight component being a copolymer of ethylene and at least one α-olefin having a density in the range 902 to 912 kg/m³; and wherein components (A) and (B) are obtainable using a Ziegler-Natta catalyst.

The densities of the higher molecular weight component are however too high to achieve demanding impact values.

WO2006037602 A1 discloses multilayer films including a core multimodal layer prepared with Borstar process and having a homopolymer low molecular weight fraction and a 1-hexene high molecular weight copolymer fraction. The HMW copolymer fraction has density lower than 915 kg/m³ and MFR₂ lower than 1 g/10 min.

WO2006/066952 describes a multimodal copolymer of ethylene comprising at least (i) a lower molecular weight fraction of a copolymer of ethylene and at least one alpha-olefin comonomer and (ii) a higher molecular weight fraction of a copolymer of ethylene and at least one alpha-olefin comonomer, wherein said copolymer has a density below 920 kg/m³.

The material has a narrow molecular weight distribution and hence a lower FRR 21/2, which can lead to difficulties in the extrusion process, leading to e.g. big number of gels and hence bad optical appearance of the films.

EP 2883885 A and EP 2883887 A disclose a method of making multimodal ethylene polymers where the method comprises polymerizing ethylene in three polymerization stages. In both cases the impact properties of the films produced, i.e. DDI is reported to be <400 g when tested on 40 μm-films.

In view of the prior art there still remains a problem for producing LLDPE polymers having a low density and which can be extruded to films having good mechanical properties, especially good impact properties combined with high E-moduli at a high throughput.

Although LLDPEs have been continuously optimized to have high impact in e.g. 40 μm films, it seems that this was often at the expense of increased low molecular fractions, i.e. increased levels of hexane soluble fractions.

It is therefore a need to find products, which can be converted into films having both high impact strength, i.e. high DDI as well as low amounts of fractions soluble in hexane.

OBJECT OF THE INVENTION

So the present inventors sought for solutions, how to provide multimodal polyethylene copolymers, which can be converted into films having good mechanical properties, especially good impact properties, expressed by a DDI₄₀ of 650 g or more when measured on a 40 μm-film, and which can be produced at a high throughput.

It is known for the skilled person that DDI can be positively influenced by

-   -   low density of the high-molecular-weight fraction combined with     -   low MFR of the high-molecular-weight fraction and     -   high amount of the low density, i.e. high-molecular-weight         fraction.

With the known polymerization settings it is well possible to achieve low GPR-densities in the range of 900 kg/m³ or lower by running the polymerization conditions with very high amounts of 1-hexene and low or even zero amounts of hydrogen. However, such polymerization settings up to now led to uncontrolled increase of the polymer MFR in the GPR and to increasing extractable contents.

Within the polymer industry it is a continuous desire to keep the amount of extractables at a lowest possible level, as extractables may have a negative effect on processability in the sense of fume creation and/or stickiness. Further the level of extractables has to be kept low in view of health-regulatory issues, commonly known as FDA-regulations.

In addition low amounts of extractable fractions are commonly seen as indication for a well running polymerization process and good activity of the catalyst.

So the inventors have sought for a solution how to provide a multimodal copolymer of ethylene having good DDI and low levels of extractable.

Surprisingly the present inventors have now identified a multimodal ethylene copolymer characterized by

-   -   low density of the high-molecular-weight fraction,     -   a low MFR of the high-molecular-weight fraction and     -   low extractables         that provides a DDI₄₀ of 650 g or more when being converted into         40 μm-films.

So in the first embodiment the present invention encompasses

a multimodal copolymer of ethylene and at least two alpha-olefin-comonomers characterized by having

a) a density of from 906 to 925 kg/m³ determined according to ISO 1183,

b) an MFR₂₁ of 10-200 g/10 min determined according to IS01133,

c) an amount of hexane soluble fraction (C6_(FDA)) of 4.50 wt. % or below,

wherein the multimodal copolymer of ethylene comprises

d) a first copolymer of ethylene comprising a first alpha-olefin comonomer having 4 to 10 carbon atoms and further characterised by comprising at least a first and a second fraction of the first copolymer of ethylene and the first alpha-olefin comonomer and e) a second copolymer of ethylene comprising a second alpha-olefin comonomer having 6 to 10 carbon atoms.

In a further embodiment the invention provides a multimodal copolymer of ethylene and at least two alpha-olefin-comonomers, characterized by a DDI₄₀ of 650 g or more when being converted into 40 μm-films.

In a further special embodiment the invention provides a multimodal copolymer of ethylene and at least two alpha-olefin-comonomers, characterized by a ratio of DDI₄₀/C6_(FDA) of 250 g/wt. % or above.

In another special embodiment the invention describes articles such as films, comprising the multimodal copolymer of ethylene and at least two alpha-olefin comonomers.

In a further special embodiment the invention describes the use of the multimodal copolymer of ethylene and at least two alpha-olefin comonomers in films or packaging applications.

DETAILED DESCRIPTION

The present invention will now be described in further detail by the examples provided below: The following definitions are herewith introduced:

The “first copolymer” is defined as the polymer produced in the first polymerization step (preferably a loop reactor). This first copolymer may comprise two or more fractions of the first copolymer. Within the meaning of the patent it is understood, that each such fraction of the first copolymer is produced in one set of reaction conditions in one reaction stage or in one polymerization reactor. The fractions are further denominated as “first fraction of the first copolymer”, “second fraction of the first copolymer”, etc.

A “first copolymer mixture” is defined as sum of all polymer fractions produced up and including the polymer produced in the first polymerization step, i.e. prepolymerization, any first or second fraction of the first copolymer.

Similarly a “second copolymer” is defined as the polymer produced in the second polymerization step, differing from the first polymerization step, preferably produced in a gas-phase-reactor. Similarly, a “second copolymer mixture” is defined as sum of all polymer fractions produced up and including the polymer produced in the second polymerization step, i.e. prepolymerization, any first or second fraction of the first copolymer and the second copolymer.

The multimodal ethylene copolymer is a copolymer of ethylene and at least two alpha-olefin comonomers, whereby the multimodal ethylene copolymer comprises a first copolymer of ethylene and an alpha-olefin comonomer having from 4 to 10 carbon atoms and a second copolymer of ethylene and an alpha-olefin comonomer having from 6 to 10 carbon atoms. Preferably the multimodal ethylene copolymer is a copolymer of ethylene and at least two comonomers selected from 1-butene, 1-hexene, and 1-octene.

It is further preferred that the multimodal ethylene copolymer is a copolymer of ethylene and exactly two comonomers selected from 1-butene, 1-hexene, or 1-octene.

Especially preferred is a multimodal ethylene copolymer comprising

-   -   a first copolymer of ethylene comprising units derived from         ethylene and 1-butene and     -   a second copolymer comprising units derived from ethylene and         1-hexene.

Even more preferred is a multimodal ethylene copolymer comprising

-   -   a first copolymer consisting of units derived from ethylene and         1-butene and     -   a second copolymer of ethylene consisting of units derived from         ethylene and 1-hexene.

The multimodal ethylene copolymer has a final density p of from 906 to 925 kg/m³, preferably 910 to 925 kg/m³ and more preferably from 913 to 923 kg/m³. The resins having densities lower than 906 kg/m³ tend to be so sticky that their production becomes problematic in a particle forming process. On the other hand, the resins having a final density of more than 925 kg/m³ do not have the required balance of properties required in the end use applications for the multimodal ethylene copolymer, such as they are not sufficiently soft and they may have a too low dart drop strength and tear strength.

The multimodal ethylene copolymer has a melt flow rate MFR₂₁ of 10-200 g/10 min, preferably from 20-150 g/10 min, like 25-100 g/min, such as 28-80 g/10 min. The resins having an MFR₂₁ of less than 10 g/10 min tend to have too high melt viscosity so that the throughput in a converting process may become restricted.

On the other hand, the resins having MFR₂₁ of more than 200 g/10 min have too low melt strength for the end use applications. In addition, the combination of a high melt index with a low density of the resin often causes the resin particles to be sticky and this causes problems in a particle forming process, such as plugging and fouling of process equipment.

In addition, the multimodal ethylene copolymer has a flow rate ratio FRR_(21/5) of from 15 to 40, preferably from 20 to 35.

The multimodal ethylene copolymer of the present invention is characterized by a low level of extractables, especially low levels of hexane solubles as determined according to the FDA method (federal registration, title 21, Chapter 1, part 177, section 1520, s. Annex B), mentioned in here as C6_(FDA). Multimodal ethylene copolymers within the scope of the invention are characterized by having C6_(FDA)-solubles of 4.50 wt. % or below, such as 3.00 wt. % or 2.50 wt. % or below.

Especially preferred are multimodal ethylene copolymers of the current inventions having C6_(FDA)-solubles of 2.20 wt. % or below.

The copolymer of the present invention is a multimodal copolymer of ethylene and at least two alpha-olefin-comonomers.

By multimodal ethylene copolymer is meant a copolymer which contains distinct components having different average molecular weights, different contents of comonomer or both.

The multimodal copolymer of the present invention can be produced by copolymerizing ethylene and at least two comonomers in two or more polymerization stages where the polymerization conditions are sufficiently different to allow production of different polymers in different stages. Alternatively, the multimodal copolymer may be produced in a single polymerization stage by using two or more different catalysts or by using a multicomponent catalyst comprising compounds of at least two different transition metals.

First Copolymer

The first copolymer of ethylene comprises ethylene and a first alpha-olefin comonomer having 4 to 10 carbon atoms, such as 1-butene, 1-hexene or 1-octene, more preferably 1-butene.

In a preferred embodiment the first copolymer consists of ethylene and 1-butene.

The first copolymer of ethylene has a melt flow rate MFR₂ of from 150-1500 g/10 min, like 150 to 1000 g/10 min, preferably from 150 to 750 g/10 min and more preferably from 200 to 600 g/10 min. Furthermore, the first copolymer has a density of from 945 to 955 kg/m³, preferably from 945 to 953 kg/m³ and most preferably from 948 to 953 kg/m³.

It is further known that high densities in the low molecular weight fraction lead to higher stiffness of the final polyethylene composition. This goes in hand with reduced impact resistance, especially DDI-performance.

It is acknowledged among experts, that the impact behaviour (especially the DDI performance) can be improved by lowering the density of the high-molecular weight fraction.

Still, when loop densities are too high, the density of the GPR has to be very low to achieve the desired final density, which can cause problems in plant operability.

So care has to be taken to choose a density range that both gives good impact behaviour but does not cause operability problems during polymerization due to stickiness.

The present inventors have found a way, to provide a multimodal copolymer of ethylene that provides good impact properties even with higher densities of the first copolymer.

The first copolymer of ethylene is ideally produced in a first polymerization stage which is preferably a slurry polymerization.

The slurry polymerization usually takes place in an inert diluent, typically a hydrocarbon diluent such as methane, ethane, propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc., or their mixtures. Preferably the diluent is a low-boiling hydrocarbon having from 1 to 4 carbon atoms or a mixture of such hydrocarbons. An especially preferred diluent is propane, possibly containing minor amount of methane, ethane and/or butane.

The ethylene content in the fluid phase of the slurry may be from 1 to about 50% by mole, preferably from about 2 to about 20% by mole and in particular from about 2 to about 10% by mole. The benefit of having a high ethylene concentration is that the productivity of the catalyst is increased but the drawback is that more ethylene then needs to be recycled than if the concentration was lower.

The temperature in the first polymerization stage is typically from 60 to 100° C., preferably from 70 to 90° C. An excessively high temperature should be avoided to prevent partial dissolution of the polymer into the diluent and the fouling of the reactor. The pressure is from 1 to 150 bar, preferably from 40 to 80 bar.

The slurry polymerization may be conducted in any known reactor used for slurry polymerization. Such reactors include a continuous stirred tank reactor and a loop reactor. It is especially preferred to conduct the polymerization in a loop reactor. In such reactors the slurry is circulated with a high velocity along a closed pipe by using a circulation pump. Loop reactors are generally known in the art and examples are given, for instance, in U.S. Pat. Nos. 4,582,816, 3,405,109, 3,324,093, EP-A-479186 and U.S. Pat. No. 5,391,654. It is thus preferred to conduct the first polymerization stage as a slurry polymerization in one or more loop reactors, more preferably in two consecutive loop reactors.

The slurry may be withdrawn from the reactor either continuously or intermittently. A preferred way of intermittent withdrawal is the use of settling legs where slurry is allowed to concentrate before withdrawing a batch of the concentrated slurry from the reactor. The use of settling legs is disclosed, among others, in U.S. Pat. Nos. 3,374,211, 3,242,150 and EP-A-1310295. Continuous withdrawal is disclosed, among others, in EP-A-891990, EP-A-1415999, EP-A-1591460 and WO-A-2007/025640. The continuous withdrawal is advantageously combined with a suitable concentration method, as disclosed in EP-A-1310295 and EP-A-1591460. It is preferred to withdraw the slurry from the first polymerization stage continuously.

Hydrogen is introduced into the first polymerization stage for controlling the MFR₂ of the first copolymer. The amount of hydrogen needed to reach the desired MFR depends on the catalyst used and the polymerization conditions. The desired polymer properties have been obtained in slurry polymerization in a loop reactor with the molar ratio of hydrogen to ethylene of from 100 to 1000 mol/kmol (or mol/1000 mol) and preferably of from 200 to 800 mol/kmol.

The first alpha-olefin comonomer is introduced into the first polymerization stage for controlling the density of the first copolymer. As discussed above, the comonomer is an alpha-olefin having from 4 to 10 carbon atoms, preferably 1-butene, 1-hexene or 1-octene, more preferably 1-butene. The amount of comonomer needed to reach the desired density depends on the comonomer type, the catalyst used and the polymerization conditions. The desired polymer properties have been obtained with 1-butene as the comonomer in slurry polymerization in a loop reactor with the molar ratio of comonomer to ethylene of from 100 to 1000 mol/kmol (or mol/1000 mol) and preferably of from 200 to 800 mol/kmol.

The average residence time in the first polymerization stage is typically from 20 to 120 minutes, preferably from 30 to 80 minutes. As it is well known in the art the average residence time T can be calculated from Equation 1 below:

$\begin{matrix} {{{Residence}\mspace{14mu}{Time}}{\tau = \frac{V_{R}}{Q_{o}}}} & {{Equation}\mspace{14mu} 1} \end{matrix}$

Where V_(R) is the volume of the reaction space (in case of a loop reactor, the volume of the reactor, in case of the fluidized bed reactor, the volume of the fluidized bed) and Q_(o) is the volumetric flow rate of the product stream (including the polymer product and the fluid reaction mixture).

The production rate in the first polymerization stage is suitably controlled with the catalyst feed rate. It is also possible to influence the production rate by suitable selection of the monomer concentration in the first polymerization stage. The desired monomer concentration can then be achieved by suitably adjusting the ethylene feed rate into the first polymerization stage.

According to the present invention, it is beneficial that the particles of the first copolymer of ethylene and the first alpha-olefin comonomer can have a narrow distribution for the residence time. This is seen to pose advantages in view of the homogeneity of the particles, namely in view of a more homogenous catalyst activity when producing the second copolymer in the subsequent gas-phase-reactor, leading to a more even distribution of the gas-phase-reactor-fraction in/around these particles and a lower amount of easily extractable low-molecular-weight fractions.

Without being bound to any theory inventors believe, that a certain minimum residence time in the first polymerization steps influences the catalyst activity in the sense, that densities can be better controlled in the subsequent gas-phase-reactor.

So the present inventors have identified a way to create a more homogenous polymer fraction of the first copolymer of ethylene and the first alpha-olefin comonomer by splitting the production process and producing the first copolymer in two consecutive polymerization stages. The polymer produced in each such polymerization stage or set of reaction conditions in one reaction stage or in one polymerization reactor is herewith denominated as “fraction of the first copolymer”, namely “first fraction of the first copolymer”, “second fraction of the first copolymer, etc.

This split production mode leads to a more homogenous residence time of the particles of the first copolymer of ethylene and the first alpha-olefin comonomer when entering the GPR and hence more uniform properties of the particles produced in the second polymerization step, i.e. gas-phase-reactor, in view of viscosity and density.

These two properties, namely viscosity and density in combination, have then decisive influence on the final properties of the final multimodal copolymer of ethylene and any articles produced thereof.

On top of that inventors have identified, that the level of extractables, namely extractables in hexane (C6_(FDA)) became lower, when the first copolymer is produced in two consecutive loop reactors.

The inventors also identified, that the more uniform properties of the particles produced in the first copolymerization step are further essential to achieve very low densities in the GPR in combination with low MFR of the second ethylene copolymer produced in the GPR.

So in a special embodiment the multimodal ethylene copolymer of the present invention comprises of a first and a second copolymer of ethylene as mentioned above, wherein the first copolymer of ethylene comprises at least a first and a second fraction of the first copolymer of ethylene and the first alpha-olefin comonomer.

These two or more fractions of the first copolymer of ethylene may be unimodal in view of their molecular weight and/or their density or they can be bimodal in respect of their molecular weight and/or their density.

It is preferred that the two or more fractions of the first copolymer are unimodal in view of their molecular weight and density.

This first and second fraction (and any further fraction) of the first copolymer of ethylene and the first alpha-olefin comonomer can be produced by any of the known process in the art.

However it is preferred, that both fractions are produced with the same technology, especially by applying the same method and polymerization settings as disclosed with the “first copolymer”-section above.

It is within the scope of the invention, that the first and the second fraction of the first copolymer of ethylene and the first alpha-olefin comonomer are present in a ratio of 4:1 up to 1:4, such as 3:1 to 1:3, or 2:1 to 1:2, or 1:1.

It is further preferred, that the two or more fractions of the first copolymer of ethylene and a first alpha-olefin-comonomer are produced in two or more consecutive reactors according to the same process and method as given further above under “First copolymer”.

For a person skilled in the art it will be clear that—when producing the first and the second fraction of the first copolymer of ethylene and the first alpha-olefin comonomer in two consecutive reactors, there can (or even has to) be a certain difference in the MFR2-values and density-values of each fraction, (and still) to be able to define them as “unimodal”.

It is hence understood within the meaning of the invention, that—given the preferred MFR-range of 150 to 1500 g/10 min—both the MFR₂ after loop 1 and after loop2 of the first copolymer of ethylene are to be within the range of 150-1500 g/10 min.

Further, the MFR₂ after loop2 can be up to double or 1.5 times the MFR₂ after loop1 or can be the same, to be still understood as “unimodal in view of MFR”.

Accordingly it is understood within the scope of the invention, that—at the density range in between 945 and 955 kg/m³—the densities of the first and after the second (and after any further) fraction of the first copolymer may differ by at most 3 kg/m³ and still be understood as having been produced with the same process condition.

Example

Loop density after Loop density after Loop 2 950 953 Same condition 954 951 Same condition 955 950 Different condition

Second Copolymer

The second copolymer of ethylene comprises ethylene and a second alpha-olefin comonomer having 6 to 10 carbon atoms, such as 1-hexene or 1-octene, more preferably 1-hexene. It is further preferred that the second alpha-olefin comonomer has more carbon atoms than the first alpha-olefin monomer.

It is further preferred that the second alpha-olefin comonomer has 2 more carbon atoms than the first alpha-olefin monomer.

In a preferred embodiment the second copolymer consists of ethylene and 1-hexene.

Without being bound to any theory the present inventors opine, that the use of alpha-olefin comonomers having 6 to 10 carbon atoms facilitates the creation of tie-molecules already at lower molecular weight than it would be possible with lower alpha-olefin comonomer such as 1-butene.

This easier formation of tie-molecules has significant benefits when it comes to mechanical properties, especially in the sense of impact behaviour, such as DDI.

The second copolymer is produced in the presence of any previously produced polymer component, i.e. in the presence of at least the first copolymer of ethylene and the first alpha-olefin comonomer, optionally any fractions of the first copolymer and the first alpha-olefin comonomer and any prepolymerization-components, forming the so-called “second copolymer mixture”.

The melt flow rate of the second copolymer mixture corresponds to the MFR or the final product. The MFR₅ is from 0.5 to 5.0 g/10 min, preferably from 0.8 to 4.0 g/10 min.

The second copolymer mixture preferably has MFR₂₁ of 10-200 g/10 min, preferably from 15-150 g/10 min, like 25-100 g/min, such as 28-80 g/10 min

Furthermore, it preferably has the flow rate ratio FRR_(21/5) of 15-40 g/10 min, preferably 20-35 g/10 min.

It is well understood for a person skilled that the density or viscosity (MFR₂₁) of the second copolymer as such cannot be measured because the second copolymer cannot be isolated out of the second copolymer mixture and from the first copolymer.

However, the MFR₂₁ of the second copolymer can be calculated by using the so called Hagström equation (Hagström, The Polymer Processing Society, Europe/Africa Region Meeting, Gothenburg, Sweden, Aug. 19-21, 1997).

$\begin{matrix} {{{Hagström}\mspace{14mu}{formula}}{{MI}_{b} = \left( {{w \cdot {MI}_{1}^{- \frac{w^{- b}}{a}}} + {\left( {1 - w} \right) \cdot {MI}_{2}^{- \frac{w^{- b}}{a}}}} \right)^{{- a} \cdot w^{b}}}} & {{Equation}\mspace{14mu} 2} \end{matrix}$

As proposed by Hagström, a=10.4 and b=0.5 for MFR₂₁. Further, unless other experimental information is available, MFR₂₁/MFR₂ for one polymer component (i.e. first copolymer or second copolymer) can be taken as 30. Furthermore, w is the weight fraction of the polymer component having higher MFR. The first copolymer can thus be taken as the component 1 and the second copolymer as the component 2. The MFR₂₁ of the second copolymer (MI₂) can then be solved from Equation 2 when the MFR₂₁ of the first copolymer mixture (MI₁) and the second copolymer mixture (MI_(b)) are known.

Preferably the second copolymer of ethylene and a second alpha olefin comonomer has an MFR₂₁ of <20 g/10 min when calculated according to Equation 2: Hagström formula

The content of the comonomer in the second copolymer is controlled to obtain the desired density of the second copolymer mixture. Typically the second copolymer mixture has a density of from 906 to 925 kg/m³, preferably 910 to 925 kg/m³, more preferably from 913 to 923 kg/m³.

The density of the second copolymer cannot be directly measured. However, by using the standard mixing rule of Equation 3 the density of the second copolymer can be calculated starting from the densities of the second copolymer mixture and the first copolymer. Then the subscripts b, 1 and 2 refer to the overall mixture b (=second copolymer mixture), component 1 (=first copolymer) and component 2 (=second copolymer), respectively. ρ_(b) =w ₁·ρ₁ +w ₂·ρ₂  Equation 3 Density mixing rule

Within the scope of the invention it is advantageous, that the density of the second copolymer is lower than the density of the first copolymer.

The density of the second copolymer has major influence on the impact behaviour of the material and consecutively of the films produced thereof. Hence the density of the second copolymer influences the DDI of the films produced.

Still care has to be taken to choose a density range that both gives good impact behaviour but does not cause operability problems during polymerization due to stickiness.

The density of the second copolymer can be at most 903 kg/m³ or lower when being calculated according to Equation 3 based on values determined according to ISO 1183.

Preferably the density is 902 kg/m³ or below, such as 897 or 894 kg/m³ or below.

So within the scope of the inventions it is preferred, that the density of the second copolymer is at least 880 kg/m³, such as 885 kg/m³. It is further preferable, that the density of the second copolymer is within the range of 880-903 kg/m³, such as 885-902 kg/m³ or 885-897 kg/m³ or 885-894 kg/m³.

The ratio (i.e. the split) between the first and the second copolymer within the final multimodal copolymer of ethylene and at least two alpha-olefin-comonomers has significant effect on the mechanical properties of the final composition.

It is hence envisaged within the scope of the invention that the second copolymer of ethylene and the second alpha-olefin comonomer forms a significant part of the polymer fractions present in the multimodal ethylene copolymer, i.e. at least 50 wt. % of the final composition, preferably 53 wt. % or more, such as 55 wt. % or more.

More preferably the second copolymer of ethylene and the second alpha-olefin comonomer may form about 60 wt. % or more, such as 65 wt. % or more of the multimodal copolymer of the present invention.

Consecutively the first copolymer of ethylene and the first alpha-olefin comonomer forms at most 50 wt. % or less of the multimodal ethylene copolymer of the current invention, preferably 47 wt. % or less, such as 45 wt. % or less. More preferably the first copolymer of ethylene and the first alpha-olefin comonomer may form about 40% or less, such as 35 wt. % of the multimodal copolymer of the present invention.

In still another embodiment the invention presents multimodal ethylene copolymers, that—when converted into films—have a very beneficial ratio of DDI₄₀ to C6_(FDA) of 250 g/wt. % or above, such as at least 300 or 350 or 400 g/wt. % or above.

Especially preferable are multimodal ethylene copolymers, that—when converted into films—have a very beneficial ratio of DDI₄₀ to C6_(FDA) of 600 g/wt. % or higher, such as 750 or higher, such as 800 or 850 g/wt. %.

As a special embodiment the current inventors have identified multimodal ethylene copolymers that have a

-   -   DDI₄₀>=650 g when converted into a film and     -   C6_(FDA)-solubles of 4.50 wt. % or below.

As a further special embodiment the current inventors have identified multimodal ethylene copolymers that have a

-   -   calculated GPR-density of 897 kg/m³ or below and     -   C6_(FDA)-solubles of 4.50 wt. % or below.

As a further special embodiment the current inventors have identified multimodal ethylene copolymers that have a

-   -   calculated GPR-density of 902 kg/m³ or below and     -   DDI₄₀ of 980 g or more when converted into a film

As a further special embodiment the current inventors have identified multimodal ethylene copolymers that have a

-   -   calculated GPR-density of 902 kg/m³ or below and     -   C6_(FDA)-solubles of 2.20 wt. % or below.

In another special embodiment the current inventors have identified multimodal ethylene copolymer characterized in that

-   -   the second copolymer has a density calculated according to         Equation 3 of 898 kg/m³ or below and which can be converted into         a film having a DDI₄₀>1500 g when converted into a film.

Prepolymerization

The polymerization steps discussed above may be preceded by a prepolymerization step. The purpose of the prepolymerization is to polymerize a small amount of polymer onto the catalyst at a low temperature and/or a low monomer concentration. By prepolymerization it is possible to improve the performance of the catalyst in slurry and/or modify the properties of the final polymer. The prepolymerization step is conducted in slurry.

Thus, the prepolymerization step may be conducted in a loop reactor. The prepolymerization is then preferably conducted in an inert diluent, typically a hydrocarbon diluent such as methane, ethane, propane, n-butane, isobutane, pentanes, hexanes, heptanes, octanes etc., or their mixtures. Preferably the diluent is a low-boiling hydrocarbon having from 1 to 4 carbon atoms or a mixture of such hydrocarbons.

The temperature in the prepolymerization step is typically from 0 to 90° C., preferably from 20 to 80° C. and more preferably from 55 to 75° C.

The pressure is not critical and is typically from 1 to 150 bar, preferably from 40 to 80 bar.

The amount of monomer is typically such that from about 0.1 to 1000 grams of monomer per one gram of solid catalyst component is polymerized in the prepolymerization step. As the person skilled in the art knows, the catalyst particles recovered from a continuous prepolymerization reactor do not all contain the same amount of prepolymer. Instead, each particle has its own characteristic amount which depends on the residence time of that particle in the prepolymerization reactor. As some particles remain in the reactor for a relatively long time and some for a relatively short time, then also the amount of prepolymer on different particles is different and some individual particles may contain an amount of prepolymer which is outside the above limits. However, the average amount of prepolymer on the catalyst typically is within the limits specified above.

The molecular weight of the prepolymer may be controlled by hydrogen as it is known in the art. Further, antistatic additive may be used to prevent the particles from adhering to each other or the walls of the reactor, as disclosed in WO-A-96/19503 and WO-A-96/32420.

The catalyst components are preferably all introduced to the prepolymerization step when a prepolymerization step is present. However, where the solid catalyst component and the cocatalyst can be fed separately it is possible that only a part of the cocatalyst is introduced into the prepolymerization stage and the remaining part into subsequent polymerization stages. Also in such cases it is necessary to introduce so much cocatalyst into the prepolymerization stage that a sufficient polymerization reaction is obtained therein.

It is understood within the scope of the invention, that the amount or polymer produced in the prepolymerization lies within 1-5 wt. % in respect to the final multimodal copolymer.

Catalyst

The polymerization is conducted in the presence of an olefin polymerization catalyst. The catalyst may be any catalyst which is capable of producing the desired ethylene polymer. Suitable catalysts are, among others, Ziegler-Natta catalysts based on a transition metal, such as titanium, zirconium and/or vanadium catalysts. Ziegler-Natta catalysts are useful as they can produce polymers within a wide range of molecular weight with a high productivity. Ziegler-Natta-catalysts are preferred within the scope of the invention.

Suitable Ziegler-Natta catalysts preferably contain a magnesium compound, an aluminium compound and a titanium compound, optionally supported on a particulate support.

The particulate support can be an inorganic oxide support, such as silica, alumina, titania, silica-alumina and silica-titania. Preferably, the support is silica.

The average particle size of the silica support can be typically from 10 to 100 μm. However, it has turned out that special advantages can be obtained if the support has median particle size from 6 to 40 μm, preferably from 6 to 30 μm.

The magnesium compound is a reaction product of a magnesium dialkyl and an alcohol. The alcohol is a linear or branched aliphatic monoalcohol. Preferably, the alcohol has from 6 to 16 carbon atoms. Branched alcohols are especially preferred, and 2-ethyl-1-hexanol is one example of the preferred alcohols. The magnesium dialkyl may be any compound of magnesium bonding to two alkyl groups, which may be the same or different. Butyl-octyl magnesium is one example of the preferred magnesium dialkyls.

The aluminium compound is chlorine containing aluminium alkyl. Especially preferred compounds are aluminium alkyl dichlorides and aluminium alkyl sesquichlorides.

The titanium compound is a halogen containing titanium compound, preferably chlorine containing titanium compound. Especially preferred titanium compound is titanium tetrachloride. The catalyst can be prepared by sequentially contacting the carrier with the above mentioned compounds, as described in EP-A-688794 or WO-A-99/51646. Alternatively, it can be prepared by first preparing a solution from the components and then contacting the solution with a carrier, as described in WO-A-01/55230.

Another group of suitable Ziegler-Natta catalysts contain a titanium compound together with a magnesium halide compound acting as a support. Thus, the catalyst contains a titanium compound on a magnesium dihalide, like magnesium dichloride. Such catalysts are disclosed, for instance, in WO-A-2005/118655 and EP-A-810235.

Still a further type of Ziegler-Natta catalysts are catalysts prepared by a method, wherein an emulsion is formed, wherein the active components form a dispersed, i.e. a discontinuous phase in the emulsion of at least two liquid phases. The dispersed phase, in the form of droplets, is solidified from the emulsion, wherein catalyst in the form of solid particles is formed. The principles of preparation of these types of catalysts are given in WO-A-2003/106510 of Borealis.

The Ziegler-Natta catalyst is used together with an activator. Suitable activators are metal alkyl compounds and especially aluminium alkyl compounds. These compounds include alkyl aluminium halides, such as ethylaluminium dichloride, diethylaluminium chloride, ethylaluminium sesquichloride, dimethylaluminium chloride and the like. They also include trialkylaluminium compounds, such as trimethylaluminium, triethylaluminium, tri-isobutylaluminium, trihexylaluminium and tri-n-octylaluminium. Furthermore they include alkylaluminium oxy-compounds, such as methylaluminiumoxane (MAO), hexaisobutylaluminiumoxane (HIBAO) and tetraisobutylaluminiumoxane (TIBAO). Also other aluminium alkyl compounds, such as isoprenylaluminium, may be used. Especially preferred activators are trialkylaluminiums, of which triethylaluminium, trimethylaluminium and tri-isobutylaluminium are particularly useful. The amount in which the activator is used depends on the specific catalyst and activator. Typically triethylaluminium is used in such amount that the molar ratio of aluminium to the transition metal, like Al/Ti, is from 1 to 1000, preferably from 3 to 100 and in particular from about 5 to about 30 mol/mol.

Films

Films comprising the multimodal copolymer of ethylene and at least two alpha-olefin-comonomers according to the present invention can be produced with several known conversion techniques, such as extrusion via blown or cast film technology, wherein blown films are preferred.

Films according to the present invention may be subjected to post-treatment processes, e.g. lamination or orientation processes.

Such orientation processes can be mono-axially (MDO) or bi-axially orientation, wherein mono-axial orientation is preferred.

The films according to the present invention can have a thickness in the range of 100 μm or below, preferably such as 80 μm or 70 μm or below, such as 60 μm or 55 μm or below.

Preferred are films having a thickness of 10 μm or higher, such as 15 μm, 20 μm or 25 μm.

Very preferred are films having film thicknesses of 10-55, such as 15-50.

Very especially preferred are films having a thickness of 10-40 μm, such as 15-35 μm. Equally preferred are films having a thickness of 30-60 μm, such as 35-55 μm, like 45-55 μm.

Films according to the present invention may be mono- or multilayer films, comprising one or more layers, like two, three or five layers, even up to seven, up to 9 or up to 12 layers. Multilayer films are preferred.

Further preferred are multilayer films with 3 or more layers, such as multilayer films with 5-7 layers.

In multilayer films comprising the multimodal copolymer of ethylene and at least two alpha-olefin-comonomers according to the present invention, the multimodal copolymer according to the present invention may be contained by at least one of the layers.

It is envisaged by the invention that the multimodal copolymer according to the present invention is also comprised by more than one layer, like two or three layers of the multilayer film. It may be also comprised by all layers of the multilayer film.

It is within the scope of the present invention, that a monolayer film may comprise 1-100 wt. % of the multimodal copolymer according to the present invention. It is preferred, that such monolayer film 10-90 wt. %, such as 30-70 wt. %, or like 40-60 wt. % or 45-55 wt. % of the multimodal copolymer according to the present invention.

It is further within the scope of the present invention, that in case of multilayer films each layer independently from the others may comprise 1-100 wt. % of the multimodal copolymer according to the present invention. It is preferred, that each layer independently from the others comprises 10-100 wt. %, such as 30-100 wt. %, or like 40-99.5 wt. % or 45-99.5 wt. % of the multimodal copolymer according to the present invention.

It is within the scope of the invention, that the films are characterized by a high DDI₄₀, i.e. a DDI₄₀ of at least 650 g or higher, such as 800 g or 980 g or above.

It is especially preferred, that the films according to the present invention have an DDI₄₀ of at least 980 g or above, such as at least 1050 g, 1200 g or 1350 g or above, such as at least 1500 g.

It is further within the scope of the invention, that the films comprising the multimodal copolymer of ethylene and at least two alpha-olefin-comonomers have low amounts of Hexane soluble polymer (C6_(FDA)), i.e. of at most 4.50 wt. % or below, such as 3.50 wt. % or below. Preferred are films comprising the multimodal ethylene copolymer of the current invention having a C6_(FDA)-level of 3.00 wt. % or below, such as 2.50 or 2.20 wt. % or below.

In a special embodiment the films in the scope of the invention have both a high DDI₄₀ and low amounts of Hexane soluble polymer (C6_(FDA)). This is reflected by the ratio of DDI₄₀/C6_(FDA). It is further within the scope of the invention, that the films comprising the multimodal copolymer of ethylene and at least two alpha-olefin-comonomers have a ratio DDI₄₀/C6_(FDA) of 250 g/wt. %, preferably of at least 300 or 350 or more, such as 400 g/wt. %.

Especially preferable are films comprising the multimodal ethylene copolymers, that have a ratio of DDI₄₀ to C6_(FDA) of 600 g/wt. % or higher, such as 750 or higher, such as 800 or 850 g/wt. % or higher.

Further within the scope of the invention are films having a DDI₄₀ of 980 g or above, wherein the calculated GPR-density of the multimodal copolymer of ethylene and at least two alpha-olefin-comonomers comprised therein is 902 kg/m³ or below.

Also within the scope of the invention are films having an C6_(FDA)-solubles of 2.20 wt. % or below, wherein the calculated GPR-density of the multimodal copolymer of ethylene and at least two alpha-olefin-comonomers comprised therein is 902 kg/m³ or below.

As a special embodiment the current inventors have identified films comprising the multimodal ethylene copolymer and have a

-   -   DDI₄₀>=650 g when converted into a film and     -   C6_(FDA)-solubles of 4.50 wt. % or below.

Further within the scope of the invention are films having a C6_(FDA)-solubles of 4.50 wt. % or below, wherein the calculated GPR-density of the multimodal copolymer of ethylene and at least two alpha-olefin-comonomers comprised therein is 897 kg/m³ or below.

Films comprising the multimodal copolymer of the current invention comprise shrink films, collation shrink films, wrap films, lamination films, etc.

The films according to the present invention are highly useful to being used in various packaging applications or for producing packaging articles, wherein applications related to food packaging are preferred.

Packaging articles comprising the multimodal copolymer of the current invention comprise bags, pouches, wrapping or collation films, and the like.

It is within the scope of the present invention that the multimodal copolymer of ethylene and two copolymers may comprise additives as used in the art, such as phenolic stabilizers, antioxidants, slip and antistatic agents, antiblock agents processing aids, colorants and the like.

EXAMPLES

Measuring Methods

Unless otherwise stated all analyses were conducted from stabilized pellet samples; mechanical properties were determined on 40 μm monolayer blown films.

The amount of hexane extractable polymer according to FDA method was determined on a cast film as defined by the FDA.

Melt Flow Rate

The Melt flow rate (MFR) was determined according to ISO 1133 at 190° C. The load under which the measurement is conducted is given as a subscript. Thus, the MFR under the load of 2.16 kg is denoted as MFR₂. The melt flow rate MFR₂₁ is correspondingly determined at 190° C. under a load of 21.6 kg and MFR₅ under a load of 5 kg.

The melt index MFR is herein assumed to follow the mixing rule given in Equation 2 Hagström formula

${MI}_{b} = \left( {{w \cdot {MI}_{1}^{- \frac{w^{- b}}{a}}} + {\left( {1 - w} \right) \cdot {MI}_{2}^{- \frac{w^{- b}}{a}}}} \right)^{{- a} \cdot w^{b}}$

As proposed by Hagström, a=10.4 and b=0.5 for MFR₂₁. Further, unless other experimental information is available, MFR₂₁/MFR₂ for one polymer component (i.e. first copolymer or second copolymer) can be taken as 30. Furthermore, w is the weight fraction of the polymer component having higher MFR. The first copolymer can thus be taken as the component 1 and the second copolymer as the component 2. The MFR₂₁ of the second copolymer (MI₂) can then be solved from equation 6 when the MFR₂₁ of the first copolymer mixture (MI₁) and the second copolymer mixture (MI_(b)) are known.

It is herewith stated, that the following expressions are to be understood as defined:

“MFR₂ loop1” is understood as the MFR of the polymer available after the first loop, comprising the “first fraction of the first copolymer” and optionally any polymer fraction produced in the prepolymerization-step (if any).

“Density Loop1” is understood as the density of the polymer available after the first loop, comprising the first fraction of the first copolymer and optionally any polymer fraction produced in the prepolymerization-step (if any).

“MFR₂ loop2” or “MFR₂ after loop2” is understood as the MFR of the polymer available after the second loop, i.e. comprising the first fraction of the first copolymer and the second fraction of the first copolymer and optionally polymer produced in any prepolymerization step (if any).

The MFR₂ of the polymer fraction produced in the second loop (i.e. the second fraction of the first copolymer) is to be calculated according to Equation 4 and denominates as “MFR₂ of the second loop”, i.e. the MFR2 of second fraction of the first copolymer. log MFR2(loop)=n*log MFI (split1)+(1−n)*log MFR(split2)  Equation 4: MFR mixing rule

“Loop density after Loop2” (or “Density Loop2) is understood as the density of the polymer available after the second loop, i.e. comprising the first fraction of the first copolymer and the second fraction of the first copolymer and optionally polymer produced in any prepolymerization-step (if any).

The density of the polymer fraction produced in the second loop (i.e. the density of the second fraction of the first copolymer) is to be calculated according to Equation 3.

“Final MFR₂₁” is understood as the MFR of the polymer available after the gas phase reactor (GPR), i.e. comprising all the polymer fractions produced in any preceding polymerization step, i.e. comprising the first fraction and the second fraction of the first copolymer, the high-molecular-weight fraction produced in the GPR and optionally polymer produced in any prepolymerization-step (if any).

-   -   “GPR MFR₂” denominates the MFR of the polymer fraction produced         in the GPR and is to be calculated according to Equation 2:         Hagström formula.

“Density of GPR (calc)” has been calculated according to Equation 3 based on the density values of the loop density after loop 2 and the final density of the polymer composition

Density

The Density of the polymer was measured according to ISO 1183-1:2004 Method A on compression moulded specimen prepared according to EN ISO 1872-2 (February 2007) and is given in kg/m³.

-   -   The density is herein assumed to follow the mixing rule as given         in Equation 3 Density mixing rule

“Density mixing rule” ρ_(b) =w ₁·ρ₁ +w ₂·ρ₂

Where ρ is the density in kg/m³, w is the weight fraction of the component in the mixture and subscripts b, 1 and 2 refer to the overall mixture b, component 1 and component 2, respectively.

Hexane Soluble Polymer (C6_(FDA))

The amount of hexane extractable polymer according to FDA method (federal registration, title 21, Chapter 1, part 177, section 1520, s. Annex B) was determined from films produced on a PM30 cast film extrusion line with about 220° C. melt temperature with L/D=20 and a screw diameter of 30 mm (feed zone 4 D long, 5.8 mm deep, compression zone 10 D long, metering zone 6 D long, 2.3 mm deep utilising a screen pack 36-400-900-400 mesh/cm².) A 200 mm die with a 0.55 to 0.60 mm die gap, screw speed: 50 r/min, and chill roll temperature of water: both rolls 40° C. (heating-cooling unit), Air gap: 0.5 mm, Air knife blower air supply: 1 bar. The film thickness is 100 μm.

The amount of hexane soluble polymer is determined according to FDA method (federal registration, title 21, Chapter 1, part 177, section 1520, s. Annex B) from the film samples prepared as described above. The extraction was performed at a temperature of 50° C. and an extraction time of 2 hours.

Dart Drop Strength (DDI)

Dart-drop is measured using ASTM D1709, method A (Alternative Testing Technique) from the film samples. A dart with a 38 mm diameter hemispherical head is dropped from a height of 0.66 m onto a film clamped over a hole. Successive sets of twenty specimens are tested. One weight is used for each set and the weight is increased (or decreased) from set to set by uniform increments. The weight resulting in failure of 50% of the specimens is calculated and reported.

DDI₄₀ denominates the Dart Drop Impact determined on a 40 μm blown film.

The ratio DDI₄₀/C6_(FDA) denominates the Dart Drop impact [g] determined on a 40 μm film divided by the amount of extractables determined [wt. %] according to FDA-procedure mentioned above.

Comonomer Determination (NMR Spectroscopy)

Quantitative nuclear-magnetic resonance (NMR) spectroscopy was used to quantify the comonomer content of the polymers.

Quantitative ¹³C{¹H} NMR spectra recorded in the molten-state using a Bruker Advance III 500 NMR spectrometer operating at 500.13 and 125.76 MHz for ¹H and ¹³C respectively. All spectra were recorded using a ¹³C optimised 7 mm magic-angle spinning (MAS) probehead at 150° C. using nitrogen gas for all pneumatics. Approximately 200 mg of material was packed into a 7 mm outer diameter zirconia MAS rotor and spun at 4 kHz. This setup was chosen primarily for the high sensitivity needed for rapid identification and accurate quantification. {[1], [2], [6]} Standard single-pulse excitation was employed utilising the transient NOE at short recycle delays of 3s {[1], [3]} and the RS-HEPT decoupling scheme {[4], [5]}. A total of 1024 (1 k) transients were acquired per spectrum. This setup was chosen due its high sensitivity towards low comonomer contents.

Quantitative ¹³C{¹H} NMR spectra were processed, integrated and quantitative properties determined using custom spectral analysis automation programs. All chemical shifts are internally referenced to the bulk methylene signal (δ+) at 30.00 ppm {[9]}.

Characteristic signals corresponding to the incorporation of 1-hexene were observed {[9]} and all contents calculated with respect to all other monomers present in the polymer. H=I _(*B4)

With no other signals indicative of other comonomer sequences, i.e. consecutive comonomer incorporation, observed the total 1-hexene comonomer content was calculated based solely on the amount of isolated 1-hexene sequences: H _(total) =H

Characteristic signals resulting from saturated end-groups were observed. The content of such saturated end-groups was quantified using the average of the integral of the signals at 22.84 and 32.23 ppm assigned to the 2s and 2s sites respectively: S=(½)*(I _(2S) +I _(3S))

The relative content of ethylene was quantified using the integral of the bulk methylene (δ+) signals at 30.00 ppm: E=(½)*I _(δ+) The total ethylene comonomer content was calculated based the bulk methylene signals and accounting for ethylene units present in other observed comonomer sequences or end-groups: E _(total) =E+(5/2)*B+(3/2)*S The total mole fraction of 1-hexene in the polymer was then calculated as: fH=(H _(total)/(E _(total) +H _(total))

The total comonomer incorporation of 1-hexene in mole percent was calculated from the mole fraction in the usual manner: H[mol %]=100*fH The total comonomer incorporation of 1-hexene in weight percent was calculated from the mole fraction in the standard manner: H[wt %]=100*(fH*84.16)/((fH*84.16)+((1−fH)*28.05))

-   [1] Klimke, K., Parkinson, M., Piet, C., Kaminsky, W., Spiess, H.     W., Wilhelm, M., Macromol. Chem. Phys. 2006; 207:382. -   [2] Parkinson, M., Klimke, K., Spiess, H. W., Wilhelm, M., Macromol.     Chem. Phys. 2007; 208:2128. -   [3] Pollard, M., Klimke, K., Graf, R., Spiess, H. W., Wilhelm, M.,     Sperber, O., Piel, C., Kaminsky, W., Macromolecules 2004; 37:813. -   [4] Filip, X., Tripon, C., Filip, C., J. Mag. Resn. 2005, 176, 239 -   [5] Griffin, J. M., Tripon, C., Samoson, A., Filip, C., and     Brown, S. P., Mag. Res. in Chem. 2007 45, S1, S198 -   [6] Castignolles, P., Graf, R., Parkinson, M., Wilhelm, M.,     Gaborieau, M., Polymer 50 (2009) 2373 -   [7] Zhou, Z., Kuemmerle, R., Qiu, X., Redwine, D., Cong, R., Taha,     A., Baugh, D. Winniford, B., J. Mag. Reson. 187 (2007) 225 -   [8] Busico, V., Carbonniere, P., Cipullo, R., Pellecchia, R.,     Severn, J., Talarico, G., Macromol. Rapid Commun. 2007, 28, 1128 -   [9] J. Randall, Macromol. Sci., Rev. Macromol. Chem. Phys. 1989,     C29, 201.

Material Description:

Catalyst Preparation

Complex Preparation:

87 kg of toluene was added into the reactor. Then 45.5 kg Bomag A (Butyloctyl magnesium) in heptane was also added in the reactor. 161 kg 99.8% 2-ethyl-1-hexanol was then introduced into the reactor at a flow rate of 24-40 kg/h. The molar ratio between BOMAG-A and 2-ethyl-1-hexanol was 1:1.83.

Solid Catalyst Component Preparation:

275 kg silica (ES747JR of Crossfield, having average particle size of 20 μm) activated at 600° C. in nitrogen was charged into a catalyst preparation reactor. Then, 411 kg 20% EADC (2.0 mmol/g silica) diluted in 555 litres pentane was added into the reactor at ambient temperature during one hour. The temperature was then increased to 35° C. while stirring the treated silica for one hour. The silica was dried at 50° C. for 8.5 hours. Then 655 kg of the complex prepared as described above (2 mmol Mg/g silica) was added at 23° C. during ten minutes. 86 kg pentane was added into the reactor at 22° C. during ten minutes. The slurry was stirred for 8 hours at 50° C. Finally, 52 kg TiCl₄ was added during 0.5 hours at 45° C. The slurry was stirred at 40° C. for five hours. The catalyst was then dried by purging with nitrogen.

Polymerization—Inventive Examples IE1-IE8

A loop reactor having a volume of 50 dm³ was operated at a temperature of 70° C. and a pressure of 63 bar. Into the reactor were ethylene, 1-butene, propane diluent and hydrogen so that the feed rate of ethylene was 2.0 kg/h, hydrogen was 5.0 g/h, 1-butene was 80 g/h and propane was 50 kg/h. Also 11 g/h of a solid polymerization catalyst component produced as described above was introduced into the reactor together with triethylaluminium cocatalyst so that the molar ratio of Al/Ti was about 15. The production rate was 1.9 kg/h.

A stream of slurry was continuously withdrawn and directed to a loop reactor having a volume of 150 dm³ and which was operated at a temperature of 85° C. and a pressure of 61 bar. Into the reactor were further fed additional ethylene, propane diluent, 1-butene comonomer and hydrogen so that the ethylene concentration in the fluid mixture was 2,6-5,2% by mole, the hydrogen to ethylene ratio was 270-1000 mol/kmol, the 1-butene to ethylene ratio was 300-3300 mol/kmol and the fresh propane feed was 41 kg/h. The production rate was 7-21 kg/h. A stream of slurry from the reactor was withdrawn intermittently and directed into a loop reactor having a volume of 350 dm³ and which was operated at 85° C. temperature and 54 bar pressure. Into the reactor was further added fresh propane feed of 69 kg/h and ethylene, 1-butene and hydrogen so that the ethylene content in the reaction mixture was 1.9-4.6 mol %, the molar ratio of 1-butene to ethylene was 490-1260 mol/kmol and the molar ratio of hydrogen to ethylene was 240-500 mol/kmol. The production rate was 13-26 kg/h.

The slurry was withdrawn from the loop reactor intermittently by using settling legs and directed to a flash vessel operated at a temperature of 50° C. and a pressure of 3 bar. From there the polymer was directed to a fluidized bed gas phase reactor operated at a pressure of 20 bar and a temperature of 80° C. Additional ethylene, 1-hexene comonomer, nitrogen as inert gas and hydrogen were added so that the ethylene content in the reaction mixture was 13-26 mol-%, the ratio of hydrogen to ethylene was 4-13 mol/kmol and the molar ratio of 1-hexene to ethylene was 190-310 mol/kmol. The polymer production rate in the gas phase reactor was 43-65 kg/h and thus the total polymer withdrawal rate from the gas phase reactor was up to 115 kg/h. The polymer powder was mixed under nitrogen atmosphere with 500 ppm of Ca-stearate and 1200 ppm of Irganox B225. Then it was compounded and extruded under nitrogen atmosphere to pellets by using a CIMP90 extruder so that the SEI was 230 kWh/ton and the melt temperature 260° C.

The detailed operation conditions in the loop reactor and the gas phase reactor are shown in Table 1 and Table 2 below.

The polymer properties are indicated in Table 3

Comparative Example 1

A loop reactor having a volume of 50 dm³ was operated at a temperature of 70° C. and a pressure of 63 bar. Into the reactor were ethylene, propane diluent and hydrogen so that the feed rate of ethylene was 2.0 kg/h, hydrogen was 4.7 g/h, 1-butene was 80 g/h and propane was 47 kg/h. Also 8.9 g/h of a solid polymerization catalyst component produced as described above was introduced into the reactor together with triethylaluminium cocatalyst so that the cocatalyst feed rate was 3.7 g/h. The production rate was 1.9 kg/h.

A stream of slurry was continuously withdrawn and directed to a loop reactor having a volume of 500 dm³ was operated at a temperature of 85° C. and a pressure of 59 bar. Into the reactor were ethylene, propane diluent, 1-butene and hydrogen so that the ethylene concentration in the fluid mixture was 6.2% by mole, the hydrogen to ethylene ratio was 184 mol/kmol, the molar ratio of 1-butene to ethylene was 223 mol/kmol and the propane feed was 40 kg/h. The ethylene copolymer withdrawn from the reactor had MFR₂ of 95 g/10 min and density of 956 kg/m³. The production rate was 29.1 kg/h.

The slurry was withdrawn from the loop reactor intermittently by using settling legs and directed to a flash vessel operated at a temperature of 50° C. and a pressure of 3 bar. From there the polymer was directed to a gas phase reactor operated at a pressure of 20 bar and a temperature of 85° C. Additional ethylene, 1-butene and/or 1-hexene comonomer, nitrogen as inert gas and hydrogen were added so that the ethylene content in the reaction mixture was 20 mol-%, the ratio of hydrogen to ethylene was 31 mol/kmol and the molar ratio of 1-butene to ethylene was 180 mol/kmol. The polymer production rate in the gas phase reactor was 47 kg/h. The polymer had a melt flow rate MFR₅ of 2.4 g/10 min and a density of 930 kg/m³. The production split (weight-% 1^(st) stage component/weight-% 2^(nd) stage component) was 39/61. The polymer was mixed with 500 ppm of Ca-stearate and 1200 ppm of Irganox B225 under nitrogen atmosphere. Then it was compounded and extruded under nitrogen atmosphere to pellets by using a CIMP90 extruder so that the SEI was 190 kWh/ton and the melt temperature 240° C.

Comparative Examples 2 to 16 (CE2-CE16)

The procedure of Comparative Example 1 was followed except that the operation conditions in the loop reactor and the gas phase reactor were modified as shown in Table 1 below. Temperature in the gas phase reactor was in the range 80-85° C.

1-butene was also added in gas phase reactor for CE10-CE16, the molar ratio of 1-butene to ethylene was 100-235 mol/kmol.

TABLE 1 Polymerization conditions in slurry phase. 1^(st) loop 2^(nd) loop C2 H2/C2 C4/C2 production C2 H2/C2 C4/C2 production conc. ratio ratio rate conc. ratio ratio rate Operation mol-% mol/kmol mol/kmol kg/h mol-% mol/kmol mol/kmol kg/h CE1 ML C6 6.2 180 220 29 — — — — CE2 ML C6 4.1 255 480 28 — — — — CE3 ML C6 3.8 260 520 28 — — — — CE4 ML C6 4 290 570 27 — — — — CE5 ML C6 4.1 285 540 27 — — — — CE6 ML C6 3.9 270 600 27 — — — — CE7 ML C6 3.7 275 560 34 — — — — CE8 ML C6 3.6 270 580 34 — — — — CE9 ML C4/6 3.8 280 500 42 — — — — CE10 ML C4/6 4 280 545 42 — — — — CE11 ML C4/6 4.1 270 500 42 — — — — CE12 ML C4/6 4.1 280 490 41 — — — — CE13 ML C4/6 4.1 270 500 42 — — — — CE14 ML C4/6 4.3 290 560 42 — — — — CE15 ML C4/6 4.3 300 640 38 — — — — CE16 ML C4/6 4.3 290 500 42 — — — — IE1 SL C6 2.6 490 355 17 4.5 260 580 24 IE2 SL C6 5.2 330 330 15 4 330 640 25 IE3 SL C6 4.9 310 340 16 4.6 300 660 23 IE4 SL C6 4.6 340 345 16 4.3 330 730 24 IE5 SL C6 4 350 375 16 3.1 390 850 26 IE6 SL C6 3.6 950 3250 7.4 1.9 490 1250 13 IE7 SL C6 3.4 400 400 21 2.1 320 690 23 IE8 SL C6 4.2 280 370 21 3 240 500 22

TABLE 2 Polymerization conditions in gas phase. C2 conc. H2/C2 ratio C4/C2 ratio C6/C2 ratio production rate split (process calc.) Operation mol-% mol/kmol mol/kmol mol/kmol kg/h wt-% CE1 ML C6 20 31 0 180 47 61 CE2 ML C6 22 35 0 200 52 65 CE3 ML C6 23 45 0 200 51 65 CE4 ML C6 17 20 0 260 40 60 CE5 ML C6 16 13 0 250 40 60 CE6 ML C6 12 0 0 210 41 60 CE7 ML C6 12 4 0 260 42 56 CE8 ML C6 12 5 0 210 41 55 CE9 ML C4/6 13 9 110 not avail. 57 57 CE10 ML C4/6 14 20 130 175 62 59 CE11 ML C4/6 14 18 125 170 63 59 CE12 ML C4/6 14 18 125 170 62 59 CE13 ML C4/6 14 18 115 165 61 58 CE14 ML C4/6 14 15 140 190 60 58 CE15 ML C4/6 14 16 140 185 60 59 CE16 ML C4/6 14 11 230 180 59 58 IE1 SL C6 16 4 0 200 60 58 IE2 SL C6 16 4 0 230 58 58 IE3 SL C6 26 6 0 300 52 56 IE4 SL C6 24 7 0 260 54 56 IE5 SL C6 19 9 0 310 64 59 IE6 SL C6 17 9 0 190 43 64 IE7 SL C6 13 13 0 not avail. 61 57 IE8 SL C6 14 12 0 not avail. 59 57

Operation:

ML indicates Mono-loop setting. i.e. the first copolymer of ethylene comprising a first alpha-olefin comonomer has been produced in one step.

SL indicates Split loop setting. i.e. the first copolymer of ethylene comprising a first alpha-olefin comonomer has been produced in two consecutive stages, so comprising a first and a second fraction of the first copolymer.

C6 indicates that the second copolymer of ethylene comprises 1-hexene as second alpha-olefin comonomer

C4/6 indicates that the second copolymer of ethylene comprises both 1 butene and 1-hexene as second alpha-olefin comonomer.

Film Production for Mechanical Testing:

The polymers were extruded to films having a thickness of 40 μm on a Windmöller and Hölscher (W&H) mono layer blown film line with a Varex E60.30D extruder, cylinder diameter of 60 mm, screw length to diameter ratio of 30, die diameter of 200 mm, die gap of 1.2 mm, BUR 1:3, frostline height of 700 mm and a throughput of 80 kg/hr utilising the following barrel temperature profile according to the MFR₅ of the material.

MFR₅>1.2-2.0: 80 160 180 180 180 180 180 180 180° C.

MFR₅>2.0-5.0: 80 150 160 160 160 160 160 160 160° C.

TABLE 3 Polymer properties Calculated GPR conditions Product Design MFR₂ Final Final Calc GPR Density Loop1 Loop2 MFR₂ GPR Final MFR₅ MFR₂₁ FRR MFR₂ GPR MFR₂₁ split Loop1 g/ split Density Loop2 Split Density g/ g/ 21/5 g/ Density g/ Campaign Operation 1-2 kg/m³ 10 min 2-2 kg/m³ g/10 min % kg/m³ 10 min 10 min [—] 10 min kg/m³ 10 min CE1 ML C6 0 0 0 956 95 61 930 2.4 38 16 0.62 913 6.4 CE2 ML C6 0 0 0 951 270 65 924 2.5 42 17 0.65 910 8.2 CE3 ML C6 0 0 0 952 270 65 925 3.4 55 16 0.88 910 11 CE4 ML C6 0 0 0 952 270 60 922 3.9 80 21 1.01 902 12 CE5 ML C6 0 0 0 952 260 60 921 2.8 57 20 0.73 900 8.0 CE6 ML C6 0 0 0 951 240 60 921 1.5 37 25 0.39 901 4.7 CE7 ML C6 0 0 0 951 230 56 921 3.5 82 23 0.91 896 9.1 CE8 ML C6 0 0 0 951 310 55 924 2.4 57 24 0.62 902 4.8 CE9 ML C4/6 0 0 0 41 951 340 57 922 1.5 36 24 0.39 900 3.1 CE10 ML C4/6 0 0 0 40 951 370 59 918 4.2 95 23 1.09 895 12 CE11 ML C4/6 0 0 0 39 948 290 59 920 2.3 53 23 0.60 901 6.6 CE12 ML C4/6 0 0 0 39 950 290 59 919 3.4 79 23 0.91 897 10 CE13 ML C4/6 0 0 0 40 950 260 58 922 2.3 49 22 0.59 902 5.6 CE14 ML C4/6 0 0 0 40 950 350 58 914 5.6 130 23 1.46 888 17 CE15 ML C4/6 0 0 0 39 948 340 59 913 5.2 120 23 1.35 888 16 CE16 ML C4/6 0 0 0 41 950 360 58 913 7.0 160 23 1.87 886 22 IE1 SL C6 16 948 180 24 950 220 58 918 2.3 55 24 0.60 895 6.8 IE2 SL C6 15 950 260 25 952 320 58 923 1.2 32 27 0.31 902 3.2 IE3 SL C6 17 952 220 25 951 240 56 922 1.0 26 26 0.26 899 2.1 IE4 SL C6 17 952 280 25 948 290 56 920 1.1 30 27 0.29 898 2.5 IE5 SL C6 15 948 370 24 949 530 59 914 2.0 53 27 0.52 889 5.4 IE6 SL C6 13 950 380 22 949 610 64 914 2.6 70 27 0.68 893 11 IE7 SL C6 20 948 410 22 949 460 57 918 2.5 54 22 0.65 894 4.8 IE8 SL C6 20 949 180 22 951 190 57 923 1.2 27 23 0.31 901 2.5

Splits: any difference to 100% is to be attributed to fractions coming from the prepolymerization

TABLE 4 Material Performance C4 C4 C6 C6 DDI₄₀ C6 FDA DDI₄₀/C6_(FDA) mol % wt. % mol % wt. % g % g/% CE1 ML C6 0.2 0.4 2.0 5.7 170 1.97 85 CE2 ML C6 0.3 0.6 2.4 6.9 200 2.16 93 CE3 ML C6 0.3 0.6 2.5 7.1 190 2.24 86 CE4 ML C6 0.4 0.7 3.0 8.6 218 4.02 54 CE5 ML C6 0.3 0.6 3.2 9.1 300 4.12 73 CE6 ML C6 0.3 0.6 3.1 8.8 630 3.09 203 CE7 ML C6 0.5 0.9 3.3 9.3 350 4.54 77 CE8 ML C6 0.5 0.9 2.6 7.5 270 2.82 96 CE9 ML C4/6 1.1 2.1 2.6 7.4 560 2.60 215 CE10 ML C4/6 N.A. N.A. N.A. N.A. 400 4.19 94 CE11 ML C4/6 N.A. N.A. N.A. N.A. 400 3.02 133 CE12 ML C4/6 1.2 2.3 3.0 8.4 460 3.84 120 CE13 ML C4/6 N.A. N.A. N.A. N.A. 300 2.54 117 CE14 ML C4/6 N.A. N.A. N.A. N.A. 760 5.84 130 CE15 ML C4/6 1.6 2.9 4.0 10.9  970 6.18 157 CE16 ML C4/6 N.A. N.A. N.A. N.A. 500 6.45 78 IE1 SL C6 0.6 1.1 2.9 8.0 860 2.90 297 IE2 SL C6 0.5 0.9 2.8 8.0 910 2.00 456 IE3 SL C6 0.5 0.9 2.8 7.8 1560 1.76 884 IE4 SL C6 0.5 1.0 2.7 7.8 1600 2.10 761 IE5 SL C6 0.6 1.1 3.8 10.6  1700 4.34 392 IE6 SL C6 N.A. N.A. N.A. N.A. 1610 4.46 362 IE7 SL C6 N.A. N.A. N.A. N.A. 1040 3.62 288 IE8 SL C6 N.A. N.A. N.A. N.A. 690 1.90 362

From the tables above it can be clearly seen that operating the polymerization in split loop mode (i.e. the first copolymer of ethylene comprising a first alpha-olefin comonomer has been produced in two consecutive stages) poses advantages when high DDI₄₀ or low extractables (i.e. low C6_(FDA)) are to be achieved. 

The invention claimed is:
 1. A film comprising a multimodal copolymer of ethylene and at least two alpha-olefin-comonomers having a) a density from 906 to 925 kg/m³ determined according to ISO 1183, b) an MFR₂₁ from 10 to 200 g/10 min. determined according to ISO 1133, c) an amount of hexane soluble fraction (C6_(FDA)) of 4.50 wt. % or below, wherein the multimodal copolymer of ethylene comprises d) a first copolymer of ethylene comprising a first alpha-olefin comonomer having 4 to 10 carbon atoms and comprising at least a first and a second fraction of the first copolymer of ethylene and the first alpha-olefin comonomer and e) a second copolymer of ethylene comprising a second alpha-olefin comonomer having 6 to 10 carbon atoms, wherein the second alpha-olefin comonomer has more carbon atoms than the first-alpha comonomer; and wherein the film has a DDI₄₀ according to ASTM D1709-A of at least 650 g and a DDI₄₀/C6_(FDA) ratio of 250 g/wt. % or higher.
 2. The film of claim 1 wherein the first and the second fraction of the first copolymer of ethylene and the first alpha-olefin comonomer are produced in two consecutive steps.
 3. The film of claim 1 wherein the multimodal copolymer of ethylene comprises c1) 50 wt. % or less of the first copolymer of ethylene d) d1) 50 wt. % or more of the second copolymer of ethylene e).
 4. The film of claim 1 wherein the first copolymer of ethylene and a first alpha olefin comonomer d) has i. a density from 945 to 955 kg/m³ and ii. a melt flow rate MFR₂ from 150 to 1500 g/10 min.
 5. The film of claim 1 wherein the second copolymer of ethylene and a second alpha olefin comonomer e) has a density less than or equal to 903 kg/m³ determined according to ISO
 1183. 6. The film of claim 1 wherein the second copolymer of ethylene and a second alpha olefin comonomer has MFR₂₁ less than 20 g/10 min determined according to ISO
 1133. 7. The film of claim 1 having hexane solubles (C6_(FDA)) of 4.50 wt. % or lower.
 8. A packaging article comprising the film of claim
 1. 9. The packaging article of claim 8, wherein the article is food packaging article.
 10. A process for the preparation of the film of claim 1 comprising a multimodal copolymer of ethylene and at least two alpha-olefin-comonomers comprising polymerising ethylene and a first alpha-olefin comonomer having 4 to 10 carbon atoms as a slurry polymerisation in two consecutive loop reactors to form a first copolymer of ethylene comprising a first alpha-olefin comonomer having 4 to 10 carbon atoms and comprising at least a first and a second fraction of the first copolymer of ethylene and the first alpha-olefin comonomer; and in the presence of the first copolymer polymerising ethylene and a second alpha-olefin comonomer having 6 to 10 carbon atoms; to form a multimodal copolymer of ethylene and at least two alpha-olefin-comonomers having a. a density from 906 to 925 kg/m³ determined according to ISO 1183, b. an MFR₂₁ from 10 to 200 g/10 min. determined according to ISO 1133, c. an amount of hexane soluble fraction (C6_(FDA)) of 4.50 wt. % or below, wherein the second alpha-olefin comonomer has more carbon atoms than the first-alpha comonomer; and extruding said multimodal copolymer of ethylene and at least two alpha-olefin-comonomers to form said film, wherein the film has a DDI₄₀ according to ASTM D1709-A of at least 650 g and a DDI₄₀/C6_(FDA) ratio of 250 g/wt. % or higher. 